1. Field of the Invention
The present invention relates to the preparation of granular rare earth mixed or simple phosphates having characteristic morphology, optionally containing additives, and to such novel rare earth phosphate particulates, per se.
This invention also relates to the use of the aforesaid novel rare earth phosphate particulates as luminophors and/or matrices for luminophors, or as precursors thereof.
By the term "rare earth(s)" are intended the elements belonging to the lanthanide group of the Periodic Table having an atomic number ranging from 57 to 71, but including yttrium which has an atomic number of 39.
2. Description of the Prior Art
The rare earth mixed or simple phosphates, having the general formula LnPO.sub.4 in which Ln represents at least one rare earth, are known useful compounds, in particular by reason of their optical, and more especially luminescence, properties.
Luminescence comprehends that phenomenon, for a product, of emitting electromagnetic waves in a given spectrum under the influence of external monochromatic excitation, which is itself of a given wavelength.
These rare earth phosphates can, moreover, also per se known to this art, optionally contain certain dopants or doping elements (Na, Li, K, Th, B, etc.) as additives for the purpose of promoting and/or further strengthening their luminescence potential.
Among the best known rare earth phosphates in the luminophor field, particularly representative are cerium phosphate, yttrium cerium phosphates, yttrium terbium phosphates and lanthanum cerium terbium phosphates.
Luminophors based on certain of these compounds are today widely used on an industrial scale, in particular in the field of lamps, such as, for example, LaCeTb phosphate.
The syntheses generally described in the literature for the preparation of rare earth mixed or simple orthophosphates may be classified overall in two principal categories, namely: (i) the so-called "dry-treatment" processes and (ii) the so-called "wet-treatment" processes.
The dry-treatment processes entail providing a rare earth oxide or mixture of the oxides of different rare earths, or, alternatively, providing a mixed oxide of said rare earths, and then phosphatizing this oxide or this mixture of oxides or this mixed oxide, by calcining in the presence of diammonium phosphate (solid/solid type reaction).
The wet-treatment processes entail a direct synthesis, in a liquid reaction medium, of a rare earth mixed or simple phosphate, or of a mixture of rare earth phosphates, this typically being carried out by attacking a solid compound containing the rare earth or earths (carbonate and/or oxide) by means of a phosphoric acid solution to precipitate the corresponding phosphates. It will be appreciated that it is also possible to carry out the direct precipitation of the phosphate from a solution of soluble rare earth salts.
Thus, in published French patent application No. 91/01,215, assigned to the assignee hereof, a process is described for the synthesis of a rare earth mixed phosphate, comprising mixing a solution of soluble lanthanum, cerium and terbium salts with phosphate ions, with control of the pH of the precipitation medium by maintaining same at a constant value above 2, and then permitting the precipitate thereby obtained to age or ripen. Although this process makes it possible, among other advantages, to satisfactorily overcome the filtration problems encountered in the conventional techniques employing direct precipitation (production of gels or precipitates which are very difficult to filter off), it nevertheless yields products whose particle size distribution may be considered to be inadequate, since the dispersion index of the particles of which both the mixed phosphate precursor and the resulting luminophor are comprised is greater 0.5.
The different processes indicated above, whether based on a dry treatment or a wet treatment, present the common major drawback of yielding rare earth phosphates possessing an uncontrolled particle size, especially a particle size distribution which is not narrow or which is, at least, insufficiently narrow. Thus, serious need continues to exist for monodisperse rare earth phosphate particles having uniform and narrowly defined particle size that are useful for the manufacture of efficacious luminophors, namely, luminophors which, on the one hand, are easy to incorporate in lamps or on television screens (coatings in fine, homogeneous layers), and, on the other, exhibit optimal luminescence, in particular brilliance. Indeed, it has now been determined that the quality of the final luminophor for a desired application is essentially and singularly dependent on the quality of the starting phosphate.